Nemesis remains one of the more enigmatic deities of DC's Olympians and has remained an unseen character for most of Wonder Woman's publication.
Nemesis has made her only appearance during the Odyssey story as the primary villain; over the years Nemesis had been feeding on the vengeance she delivered, eventually the constant demand for retribution drove her mad and beyond the control of the rest of Olympus, which she seemingly destroyed some time before Diana was born.
For most of the story, Nemesis' will is carried out by three death goddess' that collectively call themselves the Morrigan. Nemesis herself only appears after her minions defeat, to battle Wonder Woman herself, revealing that she intended to purge the world of the living to prevent vengeance from being needed again. Which was actually ruse, all Nemesis wanted, was for Diana to kill her, to take away the burden of being Nemesis, leaving Diana to replace her.
In Persona 3 Nemesis is the persona of Ken Amagi.